Adopt EventEmitter class instead of injecting Signal methods

Introduce a new class, EventEmitter, which implements signal
handling for pure JavaScript classes. EventEmitter still
utilizes GJS' addSignalMethods internally.

EventEmitter allows static typechecking to understand the
structure of event-emitting JS classes and makes creating
child classes simpler.

The name 'EventEmitter' mirrors a common name for this pattern
in Node and in JS libraries.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2043>

js/ui/keyboard.js

@@ -2,7 +2,7 @@
 /* exported KeyboardManager */
 
 const { Clutter, Gio, GLib, GObject, Graphene, Meta, Shell, St } = imports.gi;
-const Signals = imports.signals;
+const Signals = imports.misc.signals;
 
 const EdgeDragAction = imports.ui.edgeDragAction;
 const InputSourceManager = imports.ui.status.keyboard;
@@ -574,8 +574,10 @@ var KeyboardModel = class {
     }
 };
 
-var FocusTracker = class {
+var FocusTracker = class extends Signals.EventEmitter {
     constructor() {
+        super();
+
         this._rect = null;
 
         global.display.connectObject(
@@ -670,7 +672,6 @@ var FocusTracker = class {
         return rect;
     }
 };
-Signals.addSignalMethods(FocusTracker.prototype);
 
 var EmojiPager = GObject.registerClass({
     Properties: {
@@ -2055,8 +2056,10 @@ var Keyboard = GObject.registerClass({
     }
 });
 
-var KeyboardController = class {
+var KeyboardController = class extends Signals.EventEmitter {
     constructor() {
+        super();
+
         let seat = Clutter.get_default_backend().get_default_seat();
         this._virtualDevice = seat.create_virtual_device(Clutter.InputDeviceType.KEYBOARD_DEVICE);
 
@@ -2147,4 +2150,3 @@ var KeyboardController = class {
                                           keyval, Clutter.KeyState.RELEASED);
     }
 };
-Signals.addSignalMethods(KeyboardController.prototype);